The present invention relates to a technology for controlling the shifts of an automatic transmission in a vehicle such as an automobile.
Since an automotive engine has a low output torque at a low rotational speed, as is well known in the art, the gear stages to be set by the automatic transmission is determined in dependence upon the two parameters: an engine load and a vehicle speed so that the automobile may smoothly run on not only a flat road but also an uphill. FIG. 10 is a shift diagram disclosed in Japanese Patent Laid-Open No. 30558/1983. The abscissa plots the rotational speed of the output shaft dictating the vehicle speed, and the ordinate plots a degree of throttle opening dictating the engine load. Solid lines and broken lines indicate upshift lines and downshift lines, respectively. As will be seen from that those shift lines are upward to the right, the higher stage is set for the higher speed, and the lower state is set for the larger throttle opening with the deeper depression of the accelerator pedal. Here, the reason why hysteresis is established by shifting the downshift lines leftward with respect to the upshift lines is to prevent the upshifts or downshifts from being frequently caused with small fluctuations of the vehicle speed or the throttle opening.
In the shift controls according to the shift diagram of FIG. 10, therefore, if the running conditions such as the vehicle speed or the throttle opening change to others across an upshift line, a corresponding upshift is caused to a gear stage which is determined by the new running conditions (e.g., the vehicle speed or the throttle opening). If, on the contrary, the running conditions change to others across a downshift line, a corresponding downshift is caused to a gear stage which is determined by the new running conditions. As a result, each of the gear stages excepting the lowest and highest ones is held, while the running conditions are within an area defined by the upshift line and the downshift line. Moreover, the lowest gear stage is held while the running conditions are within an area having its upper limit defined by the upshift line, whereas the highest gear stage is held while the running conditions are within an area having its lower limit defined by the downshift line.
It seems from the aforementioned shift diagram that a plurality of gear stages could be set for a specific running condition by setting the hysteresis. As has been described hereinbefore, however, when a certain running condition varies to another exceeding the shift line, there is caused a shift to a gear stage determined by that shift line. The gear stage, which is set as the running condition changes over the shift line, is uniquely determined in dependence upon the vehicle speed and the throttle opening so that no room is left for selecting the gear stages. For an efficient run exploiting the engine performance sufficiently, moreover, the individual gear stages have to be sequentially set within a predetermined vehicle speed range.
Here, the shift chart of FIG. 10 exemplifies four forward stages including an overdrive stage. In case, however, more stages are required for improving the power performance, the shift lines come close to each other to make narrow the individual areas for the gear stages. If, therefore, the shift controls according to the conventional shift diagram were to be applied to an automatic transmission having more stages of shift control, even a relatively small change in the vehicle speed or the throttle opening would be one in the running conditions exceeding the shift lines so that the shifts might frequently occur to deteriorate the driving comfort and the durability.
In order to avoid the so-called "busy shift" causing the shifts frequently, it could be conceivable to effect the so-called "skip shift" to a gear stage farther of two stages. This case is followed by a problem that the gear stage set as a result of the skip shift is not always the optimum one.